gas tank cleaning

What is the best way to clean a full tank? I took mime out of the boat to inspect the suction tube and there is sludge at the bottom of the tank. How do I go about cleaning it out?

If it is a metal tank I would it replace it. Trying to clean and seal it will just leave you died in the water. Have tried to do the cleaning and sealing to many times on old cars with not much luck.

Why the sealing if it's not leaking? It just needs to be cleaned.
Go to Walmart and get a gallon of the cleaner that comes in the purple bottle. Mix it full strength, fill the fuel tank and let it sit or a day or two, swishing or shaking it. Dump the tank and take it to the car wash and do a couple of cycles fr pressure washing, take it home, let dry, and reinstall. I did this many times with diesel tanks in old sailboats and it gets them spotless. May as well replace the pickup tube also.

If it is a metal tank with any age you my have some rust.That is why you seal the tanks.

It is a plastic tank

Bob is the cleaner in the automotive isles or like where 409 is?

Yes, it is. Can't remember the name of it but it's in a purple jug.It's the best stuff I've ever used. it will even take off lime and calcium deposits from hulls and lower units.

add a ft of steel chain inside and it will break up any gunk, and the length of chain is easier to get out than ball bearings, ect...